Process for electrostatically coating nonconductive articles



April 21, 1964 D. BULGIN 3,130,067

PROCESS FOR ELECTROSTATICALLY comma moncounucnva: ARTICLES Filed June 29, 1959 lRRADlATlON 0F Mon cououcnve GROUNDED ARTICLE w ELECTROSTATIC COQTING M2 INVENTOR DOUGLAS BULGIN n'r'rorwev United States Patent 3,138,667 PROQESS FOR ELEtITROSTATiCALLY COATING NONCONDUQTIVE ARTTQLES Douglas iiulgin, Erdington, Birmingham, England, as-

signor to Duniop Rubber Company Limited, County of London, Engiand, a British company Filed June 29, 1959, Ser. No. 823,769 Claims priority, application Great Britain July 10, 1958 5 Claims. (til. 1117-47) This invention relates to electrostatic coating, and is specially concerned with the coating of insulating materials.

ln electrostatic coating the article to be coated is normally placed in proximity to a high potential electrode such as a network of wires and is grounded. Spray-producing devices are directed into the vicinity of the electrode so that the droplets or particles of coating material produced become charged and are attracted to the grounded article. As each charged droplet or particle contacts and is deposited on the article its charge is transferred to the article Whose potential thereby tends to increase. In the case of the coating of articles of conductive material these charges are immediately conducted to ground so that the potential of the article does not rise appreciably. In the case of normally non-conductive insulating materials however the charges cannot leak rapidly to ground and the resultant increase of surface potential of the article tends to repel further droplets or particles of coating material. Thus only a very thin or uneven coating can usually be obtained on insulating materials which are thus not normally satisfactory subjects for electrostatic coating.

According to the present invention a method of electrostatic coating of articles includes the step of submitting the articles to be coated to penetrating ionizing radiation in the form of w, /3-, 'y-, or X-rays immediately prior to or during their positioning in the electrostatic field.

The effect of irradiation of insulating materials is to produce free electrons or ions in the materials, which increase their electric conductivity. This increase in conductivity enables charges released during electrostatic coating of an article by coating material deposited on the article to leak to its ground connection, just as in the case of normally conducting materials.

The articles are preferably irradiated whilst they are in position in the electrostatic field, but since free elec trons remain in the material of the articles for a short time after irradiation, they may, if more convenient, be irradiated immediately prior to entering the electrostatic field. The particular arrangement chosen will depend upon the design of the electrostatic coating apparatus and the arrangement of the high potential electrodes.

The irradiation may be produced by any known means, but preferred means are either an X-ray tube or a radioactive isotope. Both these sources can conveniently be mounted in or close to the electrostatic field and their rays can be directed towards the position of the articles within the field or, in a continuous process, across the path of articles about to enter the field. The former arrangement is preferable since it enables the articles to be submitted to irradiation at the same time as they are infiuenced by the electrostatic field and thus the maximum use is made of the effect of irradiation.

As the charges from deposited coating material are released on the surface of each article, and that surface is connected to ground, the increased conductivity is required only near the surface. Thus radiations of com- 3,130,067. Fatented Apr. 21, 1964 paratively low penetration can give satisfactory results. In general therefore uor B-radiation is satisfactory.

The dose of radiation required depends upon the nature of the articles to be coated and the rate at which the coating is to be applied, since this governs the rate at which the released charges are required to be conducted away. In an example, for the painting of rubber, it was found that irradiation with fi-rays at the rate of 20 roentgens per minute increased the surface conductivity by a factor of 1,000 at room temperature and this was found to give satisfactory coating in conventional electrostatic spray painting apparatus.

The drawing shows a flow sheet of the process. In this flow sheet the irradiation is indicated by the numeral 1 and the coating by the numeral 2.

As mentioned above, a convenient apparatus for carrying out the invention comprises any conventional electrostatic coating apparatus having a radio-active source such as an X-ray tube or a radio-active isotope mounted close to or within the region of the electrostatic field of the high potential electrode or electrodes. For example any coating apparatus of the kind in which articles are carried by a travelling conveyor through the region of the electrostatic field, an X-ray tube is mounted to direct its rays onto the articles just as they enter the region of the field. Two or more X-ray tubes may be used, mounted so as to irradiate the articles during their whole passage through the region of the field.

By use of the invention a satisfactory even coating can be obtained by electrostatic coating methods on articles made from material of very low conductivity which is not normally suitable for electrostatic coating.

Having now described my invention, what I claim is:

1. A method of electrostatically coating a normally electrically non-conductive grounded article which comprises subjecting the grounded article to irradiation to render a surface of said article electrically conductive for a limited period of time, applying a high electrical potential between an electrode and said article to generate an electrostatic field, dispersing coating material in said electrostatic field while said irradiated surface remains conductive.

2. The method of claim 1 in which the non-conductive article is passed continuously through the electrostatic field.

3. The method of claim 1 in which the article to be coated is a rubber article and which is subjected to radiation at the rate of 20 roentgens per minute.

4. The method of claim 1 in which said article is subjected to irradiation immediately before being subjected to electrostatic field.

5. The method of claim 1 in which said article is irradiated while in said electrostatic field and while the coating material is dispersed in said electrostatic field.

References Cited in the file of this patent UNITED STATES PATENTS 2,334,648 Ransburg et al. Nov. 16, 1943 2,479,882 Wallhausen et al. Aug. 23, 1949 2,526,763 Miller Oct. 24, 1950 2,562,138 Bump et al. July 24, 1951 2,698,814 Ransburg Jan. 4, 1955 FOREIGN PATENTS 689,849 Great Britain Apr. 8, 1953 698,779 Great Britain Oct. 21, 1953 OTHER REFERENCES Modern Plastics, Sun, September 1954, pp. 14l-144, 146, 148, 150, 229-233 and 236-238, esp. p. 150. 

1. A METHOD OF ELECTROSTATICALLY COATING A NORMALLY ELECTRICALLY NON-CONDUCTIVE GROUNDED ARTICLE WHICH COMPRISES SUBJECTING THE GROUNDED ARTICLE TO IRRADIATION TO RENDER A SURFACE OF SAID ARTICLE ELECTRICALLY CONDUCTIVE FOR A LIMITED PERIOD OF TIME, APPLYING A HIGH ELECTRICAL POTENTIAL BETWEEN AN ELECTRODE AND SAID ARTICLE TO GENERATE AN ELECTROSTATIC FIELD, DISPERSING COATING MATERIAL IN SAID 